Re: [PATCH V14 4/7] PCI: loongson: Don't access non-existant devices

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Jun 17, 2022 at 03:43:27PM +0800, Huacai Chen wrote:
> On LS2K/LS7A, some non-existant devices don't return 0xffffffff when
> scanning. This is a hardware flaw but we can only avoid it by software
> now.

We should say what *does* happen if we do a config read to a device
that doesn't exit.  Machine check, hang, etc?

> Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
> ---
>  drivers/pci/controller/pci-loongson.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/controller/pci-loongson.c b/drivers/pci/controller/pci-loongson.c
> index a1222fc15454..e22142f75d97 100644
> --- a/drivers/pci/controller/pci-loongson.c
> +++ b/drivers/pci/controller/pci-loongson.c
> @@ -134,10 +134,20 @@ static void __iomem *cfg0_map(struct loongson_pci *priv, int bus,
>  	return priv->cfg0_base + addroff;
>  }
>  
> +static bool pdev_is_existant(unsigned char bus, unsigned int device, unsigned int function)
> +{
> +	if ((bus == 0) && (device >= 9 && device <= 20) && (function > 0))
> +		return false;

Why do you test pci_is_root_bus() below and "bus == 0" here?  I think
you intend them both to test the same thing.  If so, I think you
should test for "if (pci_is_root_bus(bus) ..." here.

Generally speaking we only probe for functions > 0 if .0 is marked as
multi-function, so I guess this means 00:09.0 is marked as a
multi-function device, but config reads to 00:09.1 would fail?

> +	return true;

Returning "true" here means "the device *may* exist," not "this device
*does* exist," right?  If so, the function name probably should be
"pdev_may_exist()".

I guess that when we do a config read to a non-root bus device that
doesn't exist, e.g., "01:00.0", that read terminates with an
Unsupported Request error, the config read gets the ~0 data we expect?

> +}
> +
>  static void __iomem *pci_loongson_map_bus(struct pci_bus *bus, unsigned int devfn,
>  			       int where)
>  {
>  	unsigned char busnum = bus->number;
> +	unsigned int device = PCI_SLOT(devfn);
> +	unsigned int function = PCI_FUNC(devfn);
>  	struct loongson_pci *priv = pci_bus_to_loongson_pci(bus);
>  
>  	if (pci_is_root_bus(bus))
> @@ -147,8 +157,13 @@ static void __iomem *pci_loongson_map_bus(struct pci_bus *bus, unsigned int devf
>  	 * Do not read more than one device on the bus other than
>  	 * the host bus.
>  	 */
> -	if (priv->data->flags & FLAG_DEV_FIX &&
> -			!pci_is_root_bus(bus) && PCI_SLOT(devfn) > 0)
> +	if ((priv->data->flags & FLAG_DEV_FIX) && bus->self) {
> +		if (!pci_is_root_bus(bus) && (device > 0))
> +			return NULL;
> +	}
> +
> +	/* Don't access non-existant devices */
> +	if (!pdev_is_existant(busnum, device, function))
>  		return NULL;

Is this a "forever" hardware bug that will never be fixed, or should
there be a flag like FLAG_DEV_FIX so we only do this on the broken
devices?

>  	/* CFG0 can only access standard space */
> -- 
> 2.27.0
>